Over the past decade, magnetic resonance imaging (MRI) has become an important medical diagnostic tools. The contrast in these images is a function of the density of proton spins in the volume of interest, and of the relaxation properties of these spins. Chemical agents which alter these relaxation properties can alter the observed contrast, and hence the distribution of these "contrast agents" has become a useful diagnostic tool. Two types of agents can be used for such purposes: chelated paramagnetic ions, how in widespread clinical use, and stable organic radicals. During the past year we have investigated the factors which govern the distribution of these agents in the rat brain. Agents were introduced intracerebrally in order to evaluate the potential diagnostic utility of this route of administration. It was found that positively charged agents including bare metal ions as well as a chelated Gadolinium- HAM complex were strongly associated with the walls of the cerebral ventricles. However, although manganese ions administered intraperitoneally rapidly crossed the blood brain carrier, the observed contrast did not indicate association with the ventricle walls. This difference indicates that manganese ions administered ip behave more like chelated ions, and are most probably complexed to transferrin. These studies have recently been extended to comparisons of the effects of Gd- DTPA with those of several stable organic radicals. Comparison of the contrast effects allow evaluation of the transmembrane flux of the organic radicals.